1. Field of the Invention
The present invention relates to an improved, defectless polycrystalline silicon semiconductor. More particularly, the present invention relates to an improved semiconductor having a stacked structure comprising a multilayered polycrystalline silicon semiconductor active layer disposed on an amorphous silicon buffer layer, which is free of defects and excelling in photoelectric conversion characteristics. The present invention includes a process for the production of said semiconductor.
2. Related Background Art
In recent years, polycrystalline silicon materials have received much public attention because they have advantages such that they have a practically acceptable electroconductivity and exhibit practically acceptable photoelectric conversion characteristics, and they can be produced at a lower production cost than that of single crystal silicon materials. Particularly, public attention has been focused on their application in the production of semiconductor devices. In fact, there have been proposed various semiconductor devices using such polycrystalline silicon materials, such as thin film transistors (hereinafter referred to as TFT), photosensor, photosensors (including photoelectric conversion elements), and photovoltaic devices (including solar cells).
Now, as for such a solar cell as a semiconductor device in which a polycrystalline silicon material is used (solar cells in which a polycrystalline silicon material is used will be hereinafter referred to as "polycrystal Si solar cell"), it is usually produced in the following manner. That is, high purity powder silicon or silicon particles are subjected to heat fusion in a mold, followed by cooling, to thereby obtain a polycrystalline silicon ingot. The resultant ingot is taken out from the mold, and sliced to obtain a plate-shaped member of a given thickness, followed by polishing, to thereby obtain a polycrystalline silicon wafer. As for the polycrystalline silicon wafer thus obtained, a pn junction is formed by diffusing dopant impurities thereinto. On the resultant wafer, a collecting electrode is formed, for instance, by means of the screen printing technique, followed by forming a reflection preventive layer. Thus, there is obtained a polycrystal Si solar cell.
According to this process, it is difficult to make the resulting polycrystal Si solar cell to be of 0.3 mm thickness or less by using such a polycrystalline silicon plate sliced from the ingot as above described. Hence, the resulting polycrystal Si solar cell unavoidably has a semiconductor active layer having a thickness that is significantly greater than that required for a semiconductor active layer of a thin film solar cell which absorbs light for photoelectric conversion, whereby there cannot be attained effective utilization of the polycrystalline silicon material. This problem eventually influences the production cost, resulting in making the polycrystalline Si solar cell costly.
The above problem in the conventional process for making a polycrystal Si solar cell apparently depends on the step of slicing an ingot and the step of polishing a member sliced from the ingot.
In order to eliminate the above problem, there have been proposed various processes for producing a polycrystal Si solar cell by forming a semiconductor active layer comprising a polycrystalline silicon thin film on a substrate made of glass or a metal such as stainless steel by an appropriate film-forming technique. The film-forming techniques include forming a polycrystalline silicon thin film directly on a glass or metal substrate by CVD, plasma CVD, or a liquid phase deposition process and forming a polycrystalline silicon thin film by first forming an amorphous silicon film or a polycrystalline silicon film of a small particle size (hereinafter referred to as small particle polycrystalline silicon film) by CVD, plasma CVD, vacuum evaporation, or a sputtering process, and subjecting said amorphous silicon film or small particle polycrystalline film to fusion or solid-phase growth treatment using laser beams or infrared rays to thereby form a polycrystalline silicon thin film. These methods are advantageous in that neither the foregoing ingot-slicing step nor the foregoing polishing step are necessary.
However, in any of these methods, there are problems in that the temperature required for the formation of a polycrystalline silicon thin film is significantly higher than room temperature and because of this, when a polycrystalline silicon thin film is formed on a glass or metal substrate which is different from the polycrystalline silicon thin film in terms of physical properties, particularly, the coefficient of thermal expansion and the coated substrate is cooled to room temperature, there is a tendency distortion to occur at the interface between the two members involved to cause defects in the polycrystalline thin film, wherein such defects prevent carriers from moving and they act as recombination centers for the carrier. Because of this, in the case of a solar cell having such a polycrystalline silicon thin film as the semiconductor active layer, solar cells with a desirable photoelectric conversion efficiency cannot be attained.